The heat exchangers designed for high fluid flow rates, for example those used in “EGR” (Exhaust Gas Recirculation) systems for reducing nitrogen oxide (NOx emissions in internal combustion engines, are devices with important masses.
The usual configuration is that of arranging a shell housing a tube bundle, which tubes form a packing. Inside the shell there circulates a first fluid, the liquid coolant, which removes heat from the tube bundle, through which tubes there circulates a second fluid, the gas to be cooled.
The tube bundle has a longitudinal shape and its mass is important. When this exchanger is subjected to external excitations as occurs in a vehicle, the bundle has a dynamic behavior which is characterized by the fundamental modes of vibration. The most relevant is the first fundamental mode of vibration and is associated with the lowest fundamental frequency. The shifts due to deformation in the antinodes of the wave associated with the mode of vibration can be high, giving rise to significant deformations which are the cause of fatigue of materials, mechanical interferences and therefore the source of breakages and noise.
In particular, truck exchangers are very long, being able to reach lengths of over 300 mm with bundles having natural frequencies of around 200 Hz, which are within the range of the engine. This type of bundles or also referred to as batteries can weigh between 5 and 11 kilograms, giving rise to very resistant supports.
The usual way of manufacturing these bundles is by means of brazing. The pipes, attachment parts and supports are assembled by interposing in the attachment surfaces brazing paste containing alloys which melt below the melting point of the component parts. The assembly thus attached is introduced in a furnace causing the melting of the brazing paste but not that of the metal of the parts to be attached. Nevertheless, although these parts do not reach a melting temperature, the temperatures of the furnace can be around 1100° C., which is sufficient to soften them. For example, a steel with martensitic structures obtained by means of tempering to achieve certain elastic properties would disappear after passing through the brazing furnace.
The bundle is formed by pipes, the natural frequency of which depends on the length thereof, the latter being reduced as said pipes are made longer. When the natural frequency of the pipes is very low and is close to the first harmonics caused by the engine, the bundle can enter into resonance and a breakage due to vibrations can occur. To prevent the problems of vibrations of the bundle of the exchanger inside the shell, support points between the bundle and the shell different from the ends are established, such that the pipes forming the bundle work as if they were shorter than they actually are, and therefore they have a higher natural frequency, far from the frequencies at which the engine vibrates. These intermediate support points therefore define new nodes which change the dynamic behavior of the bundle due to vibration.
It is convenient for these support points between the bundle and the shell to be elastic, therefore if elastic parts are incorporated for this purpose they must be installed after passing through the furnace because otherwise their elastic properties would be destroyed and any deformation that they experience would be permanent.
The solution proposed in the European patent with publication number EP1870656A2 is known. This patent application describes elastic clips which can be installed after the bundle has passed through the furnace.
The clips described are installed in one of the faces of the tube bundle, either because an anchor receiving the elastic element has previously been fixed on the bundle (in the furnace for example), or the part which keeps the tubes attached at certain points of the length of the bundle is used. This part is usually referred to as baffle.
When this part is used, holes are made which admit a flat bar from which a wedge- or harpoon-shaped element emerges. This configuration allows the insertion by sliding the elastic support in a direction parallel to the main longitudinal direction of the bundle and transverse to the part. To be removed, it is necessary to press the flexible wedge- or harpoon-shaped section and force the exit.
All the proposed solutions require the insertion in each of the faces of the bundle in an independent manner and although the anchor prevents the exit thereof it does not assure an attachment without vibrations. The wedge or harpoon recovers its position after the insertion if it has a certain clearance. This clearance is maintained after the insertion and can give rise to unwanted vibrations.
The present invention proposes a simpler alternative solution which allows its installation once the bundle has already passed through the furnace, drastically reduces the installation time and also allows its removal for its replacement in a quick manner.